1. Field of the Invention
The present invention relates to zoom lenses and, more particularly, to high-range zoom lenses suited to photographic cameras, video cameras or electronic still cameras. Still more particularly, the invention relates to zoom lenses having a range of 6 to 9 with a field angle for the wide-angle end of 60 to 75 degrees and an F-number of 4 to 6, while still maintaining a good stability of optical performance throughout the entire zooming range and throughout the entire focusing range.
2. Description of Related Art
For the objective system of the photographic camera or video camera, there has been a demand that the zoom lens to be used is of high range with high contrast and high optical performance being maintained stable over the entire zooming and focusing ranges.
In Japanese Laid-Open Patent Application No. Sho 63-195618, there has been proposed a zoom lens which comprises five lens units of positive, negative, positive, positive and negative refractive powers in order from the object side, wherein the focusing provision is made in one of the lens units other than the first lens unit. Also, in Japanese Laid-Open Patent Application No. Sho 58-179810, there has been proposed a zoom lens which comprises four lens units of positive, negative, positive and positive refractive powers in order from the object side, wherein the fourth lens unit is divided into two parts, i.e., a lens sub-unit of positive refractive power and a lens sub-unit of negative refractive power and the lens sub-unit of negative refractive power is made movable for focusing.
In Japanese Laid-Open Patent Application No. Hei 4-70708, there has been proposed a zoom lens which comprises five lens units of positive, negative, positive, positive and negative refractive powers in order from the object side, with the field angle for the wide-angle end being about 75 degrees and the zoom ratio being about 7.
In general, the zoom lenses of the 5-unit type comprising five lens units of positive, negative, positive, positive and negative refractive powers in order from the object side have a feature that, as widening of the field angle for the wide-angle end approaches 60 to 75 degrees or so and the zoom ratio increases to 6 to 9 or so, the difficulty of maintaining a good stability of high optical performance throughout the entire zooming range and throughout the entire focusing range increases rapidly. Therefore, it becomes very much important to set forth appropriate rules of design for the optical parameters of each of the lens units constituting the entire lens system.
For example, the relation in which to move the lens units during zooming, the refractive power of every lens unit, the construction and arrangement of the constituent lenses of those lens units which vary the focal length, which one of the lens units to select for the focusing purposes and the construction and arrangement of the constituent lenses of the focusing lens unit, must be appropriately determined. Otherwise, the ranges of variation of all aberrations during zooming and during focusing would increase greatly, thus increasing the difficulty of obtaining images of high quality.